Gastrulation is the earliest morphogenetic process during embryogenesis and takes place in all organisms, including humans, that go through embryonic development. The process involves highly coordinated cell movements and results in the formation of the three germ layers. The pursuit of molecules required for this process will lead to a better understanding of not only the very important developmental program, but also the control of cell movements in other events such as neural tube formation and cancer cell metastasis. This application proposes to use Drosophila melanogaster, the common fruit fly, to study gastrulation because of the availability of sophisticated molecular genetic techniques. In Drosophila, mutations in the snail gene cause defects in ventral cell invagination and the loss of mesodermal derivatives such as muscles, heart, fat bodies, and lymph gland. Snail protein functions as a transcriptional repressor to prevent the expression of lateral neuroectodermal genes in the ventral presumptive mesoderm. This allows the establishment of mesodermal cell fate. However, recent results reveal that the derepression of neuroectodermal genes, although disrupts mesodermal cell fate, does not interfere with invagination. The question of how the early patterning leads to the invagination of the presumptive mesoderm remains unanswered. This proposal plans to test the hypothesis that Snail can regulate another set of ventral genes that are required for gastrulation. Such postulation is corroborated with the findings that Snail also participates in CNS and wing disc development. Therefore, Snail may regulate multiple sets of target genes in multiple developmental processes. The specific aims of this proposal include: 1) dissection of the Snail protein using transgenic flies, tissue culture, and by sequencing mutant alleles to identify the domain required for gastrulation; 2) analysis of ventral genes to define the promoter elements that may interact directly with Snail, and isolation of novel target genes through DNA-binding one-hybrid screen in yeast; 3) the identification of downstream gastrulation genes using second site enhancer genetic screens in flies.